In a wireless communication system, a sending terminal and a receiving terminal use multiple antennae to obtain a higher speed in a manner of spatial multiplexing. Compared to general spatial multiplexing manner, an enhanced technology is one that the receiving terminal feeds channel information back to the sending terminal which uses some transmission precoding technologies according to the channel information acquired, thus improving the transmission performance significantly. For single-user Multi-Input Multi-Output (MIMO), precoding is performed by directly using channel characteristic vector information; while for multiple-user MIMO, more precise channel information is required.
The channel information is fed back mainly through a simple single codebook feedback method in the Long Term Evolution (LTE), while the performance of the MIMO transmission precoding technology depends more on the feedback accuracy of the codebook adopted.
In the related art, the basic principle of the quantized feedback of the codebook-based channel information is as follows.
Assuming that a limited feedback channel capacity is B bps/Hz, then the number of available code words is. N=2B. The characteristic vector space of a channel matrix constructs a codebook space ={F1, F2, . . . FN} through quantization. The sending and receiving terminals store the codebook together or generate the codebook in real time (the sending and receiving terminals use the same codebook). For a channel estimate value H realized for the channel each time, the receiving terminal selects, from , a code word {circumflex over (F)} most matching with the channel according to certain rules and feeds the serial number i of the code word back to the sending terminal. The serial number of the code word is called Precoding Matrix Indicator (PMI) herein. The sending terminal finds the corresponding precoding code word {circumflex over (F)} according to the serial number i of the code word, thus obtaining the corresponding channel information, wherein {circumflex over (F)} represents the characteristic vector information of the channel.
Generally, the codebook space  can be further divided into the codebook corresponding to multiple Ranks (number of layers), wherein each Rank corresponds to multiple code words for quantizing the precoding matrix composed of the channel characteristic vectors under the Rank. Since the Rank of the channel is equal to the number of non-zero characteristic vectors, there will be N columns of code words when the Rank is N. Therefore, the codebook space  can be divided into multiple sub-codebooks according to the Rank, as shown in Table 1.
TABLE 1Schematic table of dividing the codebook into multiple sub-codebooksaccording to the Rank number of layers υ (Rank)2. . . N  1  2. . .   Nset of codeset of codeset of code wordword vectors withword matrixesmatrixes with N columns1 columnwith 2 columns
Wherein, when the Rank>1, all the code words required to be stored are in the form of matrix. The codebook in the LTE protocol uses this codebook quantization feedback method. The codebook for downlink 4 transmitting antennae in the LTE is as shown in Table 2. In fact, the precoding codebook in the LTE has the same meaning with the channel information quantization codebook. For uniformity's sake, the vector can be regarded as a matrix with a dimension of 1 in this application.
TABLE 2Schematic table of codebook for downlink 4 transmitting antennae in LTECodebookTotal number of layers υIndexun12340 u0 = [1 −1 −1 −1]T W0{1} W0{14}/{square root over (2)} W0{124}/{square root over (3)} W0{1234}/21 u1 = [1 −j 1 j]T W1{1} W1{12}/{square root over (2)} W1{123}/{square root over (3)} W1{1234}/22 u2 = [1 1 −1 1]T W2{1} W2{12}/{square root over (2)} W2{123}/{square root over (3)} W2{3214}/23 u3 = [1 j 1 −j]T W3{1} W3{12}/{square root over (2)} W3{123}/{square root over (3)} W3{3214}/24 u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]T W4{1} W4{14}/{square root over (2)} W4{124}/{square root over (3)} W4{1234}/25 u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1} W5{14}/{square root over (2)} W5{124}/{square root over (3)} W5{1234}/26 u6 = [1 (1 + j)/{square root over (2)} −j (−1 + j)/{square root over (2)}]TW6{1} W6{13}/{square root over (2)} W6{134}/{square root over (3)} W6{1324}/27 u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1} W7{13}/{square root over (2)} W7{134}/{square root over (3)} W7{1324}/28 u8 = [1 −1 1 1]TW8{1} W8{12}/{square root over (2)} W8{124}/{square root over (3)} W8{1234}/29 u9 = [1 −j −1 −j]TW9{1} W9{14}/{square root over (2)} W9{134}/{square root over (3)} W9{1234}/210u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)}W10{123}/{square root over (3)}W10{1324}/211u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)}W11{134}/{square root over (3)}W11{1324}/212u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/213u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/214u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/215u15 = [1 1 1 1]TW15{1}W15{12}/{square root over (2)}W15{123}/{square root over (3)}W15{1234}/2
In the above, Wn=I−2ununH/unHun, I is an identity matrix, Wk(j) represents the vector of Column j of Matrix Wk, and Wk(j1,j2, . . . jn) represents the matrix composed of Columns j1, j2, . . . jn of Matrix Wk.
The above is the basic principle of the codebook feedback technology. In the practical application of the system, some specific parameters may be involved. In the LTE standard, the minimum feedback unit of the channel information is a Subband; one Subband is composed of several resource blocks (RB) which consists of multiple resource elements (RE), wherein an RE is the minimum unit of the time frequency resources in the LTE system. The resource expression method of LTE is still used in the LTE-A. The object of the channel information feedback of the user equipment can be anyone of the Subband, multiple Subbands (Multi-Subband) and Wideband.
The feedback of the channel state information comprises: a Channel Quality Indication (CQI), a Precoding Matrix Indicator (PMI) and a Rank Indicator (RI).
PMI represents characteristic vector information, and it is sent to the base station to be used for the downlink precoding technology.
RI is used to describe the number of the space independent channels, and corresponds to the Rank of a channel response matrix. In open-loop and closed-loop spatial multiplexing modes, the RI information is required to be fed back by the UE, while in other modes, the RI information is not required to be fed back. The Rank of the channel matrix corresponds to the number of layers.
CQI is an indication for evaluating the quality of the downlink channel. In the 3GPP 36-213 protocol, CQI is expressed by the integral values within 0˜15, representing different CQI levels respectively, wherein different CQIs correspond to their own Modulation Codes and coding rates (Modulate Code format Set, MCS). The CQI can be fed back together with the PMI.
With the development of the communication technology, the LTE-Advance system has a higher requirement for the frequency spectrum efficiency. Therefore, the number of antennae is increased to 8. At present, the codebook for 4 antennae in the LTE system can not realize the channel information feedback in the LTE-A using 8 antennae.